The present invention relates to the direct reduction of metal oxides in rotary kilns using solid carbonaceous materials as the source of fuel and reductant, and more particularly to a method and means for constructing the kiln interior to maximize the kiln output for a given volume kiln.
The interior of a direct reduction rotary kiln may be divided essentially into two operating zones, a preheat zone at the kiln feed end, wherein the materials entering the charge bed are preheated to bring them up to a temperature level at which reduction will begin, and a reduction zone, wherein the metal oxides are actually reduced to a metallic state before passing out of the kiln discharge end. The heat transfer requirements from the burning freeboard gases to the charge bed in the two zones differ substantially since in the preheat zone the need is primarily for sensible heat to raise the bed temperature to the threshold level for reduction, while in the reduction zone the reactions bringing about reduction are strongly endothermic, and create an added heat demand which varies along the length of the charge bed. The amount of heat transfer to the bed in the reduction zone therefore must generally be much higher than in the preheat zone to achieve a high level of metallization. Accordingly, to maximize the use of the kiln volume, it would seem desirable to maintain a high temperature throughout the kiln for a rapid preheating of the charge in the preheat zone, particularly when the feed materials are fed at ambient temperature, and for accelerated and maximum reduction of the oxides in the reduction zone. However, problems are presented when attempting this approach by various phenomena which occur in the charge bed. For example, when the metal oxides used are those in iron ore, rapid increases in bed temperatures in the preheat zone can cause excessively rapid phase changes in the metal oxides from hexagonal crystal hematite to cubic crystal magnetite and excessive decrepitation of the ore. Also, rapid heat up may cause the formation of sticky phases in the bed in the transition region of the kiln just beyond the preheat zone. These phases can result in sintering and uncontrolled accretion formation on the kiln walls in the transition region. Further, if certain coals are used as the carbonaceous material, rapid heat up may plasticize the coal, thereby retarding mixing of the materials in the charge bed. Consequently, the temperature levels in the kiln and heat transfer to the bed must be carefully controlled to produce and maintain a suitable temperature profile in the charge bed that will permit optimum metallization of the metal oxides while avoiding charge decrepitation, sintering, wall accretions, and other deleterious effects. A particular temperature profile for this purpose is described in U.S. Pat. No. 4,304,597, assigned to the same assignee as the present invention.
It, therefore, appears that a gradual bed temperature increase is desirable; but, unfortunately, in a kiln of a given volume, if the temperature of the charge bed in the preheat zone is brought up gradually to avoid the previously-mentioned deleterious effects, too much of the kiln volume may be used in preheating, leaving insufficient volume for the reduction zone. This will result in a low kiln output for the total operating volume of the kiln, or, in other words, inefficient operation. An actual example of such a situation is described in the ISS-AIME Ironmaking Proceedings, Volume 35, St. Louis, 1976, pp. 396-405.
To solve this latter problem, prior art solutions have included heating of the charge materials prior to feeding them to the kiln and mechanically complex techniques for rapid preheating of the charge in the kiln, such as by the use of under-bed combustion-air injection. These solutions nevertheless do not greatly decrease the risk of kiln accretions or ringing in the transition region at the start of the reduction zone, the region of the kiln which, in the absence of proper heat transfer or bed temperature control, is the most susceptible to ringing.
The present invention provides a solution to the problem of properly transferring an adequate supply of heat to the charge bed in the preheat zone of the kiln by utilizing the intermediate interior dam, used in some direct reduction and certain other process kilns, in an improved manner. This solution obviates the need for prior heating of the charge outside of the kiln, complicated rapid preheating techniques, or excessive gas temperatures, by maximizing the degree of kiln volume filling and charge residence time and, consequently, the product throughput in a kiln of a given volume.